The colour of the fillet is due to the salmon’s ability to store the pigment astaxanthin in its muscle. This pigment is naturally produced by marine algae, is a powerful antioxidant, and can be used for synthesis of vitamin A – which is vital for the immune system, skin and mucous membranes, fat metabolism, and vision.

Both astaxanthin and vitamin A are added to salmon feed to ensure there is enough to guarantee good health and growth. The pink colour of the salmon fillet is also important for making it look appealing for the consumer.

“However, the needs for vitamins and antioxidants may change when salmon are exposed to stress, and we still do not know enough about how the environment in which the salmon lives affects the colour of the fillet,” said Trine Ytrestøyl, senior scientist at Nofima, in a press release.

Stress from sea lice treatment

Stress leads to the formation of reactive compounds in the body that can damage cells. Antioxidants protect living organisms from damage by neutralising such compounds. Salmon exposed to stress may therefore have an increased need for antioxidants such as astaxanthin.

Salmon are subjected to considerable stress during their lifetime. Frequent treatments against salmon lice are often necessary. During treatment, the fish are crowded together, and the oxygen level in the water drops and this is extremely stressful for the salmon.

“When we examined data from commercial productions we observed that salmon that had undergone many treatments for sea lice had paler filets”, explained Ytrestøyl.

Ingredients in the feed can affect the colour of the fillet.

“We have previously seen that the marine omega-3 fatty acids EPA and DHA have a positive effect on fillet colour. These fatty acids are reduced in the feed when marine feed ingredients are replaced with plant-based ingredients”, Ytrestøyl explained.

Vitamin A is also an antioxidant that is abundant in marine food chains. When the feed for salmon has shifted from mainly marine raw materials to plant-based ingredients, the vitamin A content in the feed has also decreased.

“It is possible that salmon will produce more vitamin A from astaxanthin when there is little vitamin A in the feed, but the effect of this on the astaxanthin content in the fillet has not been studied in detail,” Ytrestøyl explained. Nor has it been investigated how stress affects fillet colour.

Methodology 

In the study, salmon were given feed containing three different concentrations of vitamin A and two concentrations of astaxanthin. The levels tested reflected the variation found in commercial salmon feeds. The salmon were exposed to stress by being crowded together while the oxygen level in the water was reduced, similar to sea lice treatment. This was done several times a week for five weeks.

When the salmon were not stressed, the highest level of vitamin A in the feed had a negative effect on the fillet colour. Less astaxanthin was absorbed in the intestine and digested when there was a lot of vitamin A in the feed. This may be a form of regulation to protect the salmon from excessively high levels of vitamin A, which has been shown to be harmful.

The salmon exposed to stress had less astaxanthin in the fillet compared to the control group. But there was one exception: salmon that had received feed with a high content of both vitamin A and astaxanthin did not develop paler fillets as a result of stress.

Thus, there was a different effect of high vitamin A content in the feed when the salmon were stressed compared to when they were not. This may indicate that the requirements for vitamins and antioxidants change when salmon are exposed to a stressful environment.

Key takeaways

The research results show that exposure to stress reduces fillet colour in salmon, and that the need for antioxidants may change in a stressful environment.

“Stress should be reduced as much as possible, but it cannot be completely eliminated in aquaculture. To ensure the fish have good health and quality, it is important that the amount of vitamins and antioxidants in the feed is adjusted to the challenges the fish face,” concluded Ytrestøyl .

The results are from the project “Dietary Factors and Physiological Mechanisms Interact and Control Pigmentation of Salmon Muscle,” which is funded by the Norwegian Seafood Research Fund. This project runs in collaboration with NTNU – Norwegian University of Science and Technology, and Skretting.